1. Field of the Invention
The invention disclosed herein is generally related to turbocharger systems for internal combustion engines, and in particular, to multi-stage turbochargers having bypass systems for diverting exhaust gas flow around a first turbine.
2. Description of the Related Art
Turbocharging systems, such as for use with internal combustion engines, are well known in the art. A turbocharger comprises an exhaust gas turbine coupled to an intake charge compressor. The turbine operates by receiving a stream of exhaust gas from an internal combustion engine and converting a portion of the energy in the exhaust gas stream into mechanical energy by passing the exhaust stream over blades of a turbine wheel, and thereby causing the turbine wheel to rotate. This rotational motion is then utilized by a compressor, coupled by a shaft to the turbine wheel, to compress a quantity of air to a pressure higher than air entering at its inlet, which then provides an increased amount of air available to be drawn into the internal combustion engine cylinders during the engine's intake stroke. The additional compressed air (boost) taken into the cylinders can allow more fuel to be burned within the cylinder, and thereby offers the opportunity to increase the engine's power output.
The turbine in a turbocharger is sometimes referred to as a gas expander. This is because the turbine essentially converts some of the energy represented by a pressure differential between gas in the exhaust stream and ambient pressure into mechanical energy, in the form of rotation of the turbine and compressor. As the gas in the exhaust stream loses pressure, it expands and loses potential energy.
In a turbocharged internal combustion engine system, the wide range of speed and power output levels at which the internal combustion engine may operate presents challenges for designing an appropriately matched turbocharging system with good mechanical efficiency for working with the engine. For example, while smaller turbochargers provide boost quickly and more efficiently at lower engine speeds, larger turbochargers provide boost more effectively at higher engine speeds. Because of the relatively narrow flow range over which a turbocharger operates efficiently, relative to the broad flow range generated by internal combustion engines, it is known in the prior art (e.g., in cases of high boost need), to provide a multi-stage turbocharging system, involving both a smaller (i.e., “high-pressure”) turbocharger and a larger (i.e., “low pressure”) turbocharger, wherein the smaller high-pressure turbocharger operates at lower engine speeds and the larger low pressure turbocharger takes over at higher engine speeds. It has been found valuable to switch between the two turbocharging stages through use of a bypass system to divert exhaust gas flow around the high-pressure turbocharger to the low-pressure turbocharger as needed.
Bypassing exhaust flow around a turbine is also well known in the art. Typically, turbine bypass systems are used in the prior art primarily to regulate system pressure across the high-pressure turbine, and can be operated by selectively bleeding off a portion of the upstream exhaust gas over a pressure drop through a bypass channel when backpressure caused by the turbine's operation causes the system pressure upstream of the high-pressure turbine to exceed desired levels. Bleeding of the exhaust gas to the bypass channel is generally controlled by a small regulating valve called a “wastegate” positioned in the bypass channel around the turbine. A typical wastegate valve operates somewhat like a trap door, opening a port in the bypass channel, upstream of the high-pressure turbine inlet to divert a portion of the exhaust flow around the turbine, with the bypassed exhaust flow naturally expanding over the pressure drop at the wastegate and in the passage through the bypass channel and then reuniting with the remaining exhaust flow downstream of the bypassed turbine.
FIG. 1 diagrammatically shows an internal combustion engine system with a multi-stage turbocharging and bypass system according to known art. Referring to FIG. 1, ambient air enters the system through intake line 11. The intake air may optionally be mixed with recirculated exhaust gas (EGR) to form a charge-air mixture. The ambient air or charge-air mixture flows through and is compressed by a first-stage low pressure air compressor 12.
After compression in compressor 12, the intake air may flow through a second-stage high-pressure air compressor 16 for further compression. Alternatively, the intake air may be diverted at port 13 to optional bypass channel 14 and returned to the intake line at port 17, as regulated by adjusting the opening or closing of optional bypass valve 15.
Intake air then enters the intake manifold 18 and into combustion chambers 20 of engine 19 through conventional valves (not shown) in a conventional manner. Following combustion in the combustion chambers 20, the warm, pressurized exhaust gases leave the combustion chambers 20, at a first, higher, exhaust gas energy level, through conventional valves (not shown) in a conventional manner, and flow from engine 19 through exhaust manifold 21 to exhaust channel 28.
After leaving the exhaust manifold 21, exhaust gas in exhaust channel 28 may flow through a high-pressure turbine 25. High-pressure turbine 25 in exhaust channel 28 is coupled to the high-pressure air compressor 16 in the intake line 11 through shaft 29′, and together the turbine 25 and compressor 16 form a high-pressure turbocharger 30. A portion of the exhaust gas may be selectively diverted at port 22 to bypass channel 23 and returned to the exhaust line at port 26, as regulated by opening or closing of port 22 through operation of wastegate valve 24, which is operated (actively or passively) to open in response to system pressure buildup upstream of turbine 25.
Downstream of turbine 25, the exhaust gas at a second, lower exhaust gas energy level then flows through low pressure turbine 27 for further expansion, and then exits via exhaust channel 28. Turbine 27 in exhaust channel 28 is coupled to low pressure air compressor 12 in intake line 11 through shaft 29, and together the turbine 27 and compressor 12 form a low pressure turbocharger 31.